Vapour compression systems, such as refrigeration systems, air condition systems or heat pumps, normally comprise a compressor, a condenser, an expansion device, and an evaporator arranged along a refrigerant path. Refrigerant circulates the refrigerant path and is alternatingly expanded and compressed, and heat exchange takes place in the condenser and the evaporator. Expanded refrigerant enters the evaporator in a mixed state of gaseous and liquid refrigerant. As the refrigerant passes through the evaporator, it evaporates while exchanging heat with a secondary fluid flow, such as an air flow, across the evaporator. In order to utilise the potential refrigerating capacity of the evaporator to a maximum extend, it is desirable that liquid refrigerant is present along the entire length of the evaporator. On the other hand, it is undesirable that liquid refrigerant passes through the evaporator and into the suction line, since it may cause damage to the compressor if liquid refrigerant reaches the compressor. It is therefore desirable to control the supply of refrigerant to the evaporator in such a manner that the boundary between mixed phase refrigerant and gaseous refrigerant is exactly at the outlet of the evaporator.
In order to obtain this, the superheat of the refrigerant leaving the evaporator is often measured and/or calculated. The superheat is the difference between the temperature of the refrigerant leaving the evaporator and the dew point of the refrigerant leaving the evaporator. A low superheat value, thus, indicates that the temperature of the refrigerant leaving the evaporator is close to the dew point, while a high superheat indicates that the temperature of the refrigerant leaving the evaporator is significantly higher than the dew point, and that a significant part of the evaporator therefore contains gaseous refrigerant. Thereby a significant part of the potential refrigerating capacity of the evaporator is used for heating gaseous refrigerant rather than for evaporating liquid refrigerant. It is then attempted to control the supply of refrigerant to the evaporator in such a manner that the superheat is maintained at small, but positive, level.
In order to obtain the superheat of refrigerant leaving the evaporator, the temperature as well as the pressure of the refrigerant leaving the evaporator is normally measured. As an alternative, the temperature of refrigerant leaving the evaporator and the temperature of refrigerant entering the evaporator may be measured. Thus, it is necessary to use two different sensor devices in order to obtain the superheat. This adds to the manufacturing costs of the vapour compression system. Furthermore, in the case that a pressure sensor is applied for measuring the pressure of the refrigerant leaving the evaporator, there is a risk that the pressure sensor falls out or malfunctions, thereby making it impossible to measure the superheat and preventing proper control of the supply of refrigerant to the evaporator, until the pressure sensor is restored.
U.S. Pat. No. 4,893,480 discloses a refrigeration cycle or an air conditioner including a refrigeration cycle subjected to feedback control based on modern control theory. A control output from a controlled object includes a temperature of air at a point downstream of the evaporator or a superheat of refrigerant. The superheat is derived from measurements of the temperature of refrigerant entering the evaporator and the temperature of refrigerant leaving the evaporator.